Process for the preparation of(R)-Lacosamide

ABSTRACT

The present invention is directed towards an improved, five step method for the preparation of the anti-epileptic drug (R)-Lacosamide, as illustrated in FIG.  2 . The active form of the drug is (R)-enantiomer and the present method gives high yields of (R)-enantiomer of lacosamide. The method does not involve use of any unnatural amino acids as starting material or use of protection/deprotection strategies, strong acids or hydrogenation. Instead, the method uses a cheap and easily available racemic butadiene monoepoxide as the starting material.

FIELD OF INVENTION

The present invention relates to the field of chemical synthesis. More specifically the present invention is directed towards an improved process for the synthesis of the anti-epileptic drug (R)-lacosamide.

BACKGROUND OF THE INVENTION

Epilepsy is a common chronic neurological disorder characterized by recurrent unprovoked seizures. These seizures are transient signs and symptoms of abnormal, excessive or synchronous neuronal activity in the brain. Epilepsy thus refers to a clinical phenomenon rather than a single disease entity, since there are many forms and causes of epilepsy. Various methods are available for treating epilepsy including drugs.

The present invention is restricted to anti-epileptic drugs and more specifically to an improved process for the synthesis of the popular anti-epileptic drug viz. (R)-Lacosamide, developed from functionalized amino acids. Functionalized amino acids (FAAs) are an advanced novel class of anticonvulsant agents, from which (R)-Lacosamide emerged as one of the best antiepileptic drugs for the treatment of partial-onset seizures in patients with epilepsy and as add-on treatment in brain tumor patients.

The Structure of FAAs and Lacosamide is Given in FIG. 1 (A and B).

The precise mechanism of action of (R)-Lacosamide in humans has not yet been fully elucidated, but it enhances the slow inactivation of voltage-gated sodium channels, resulting in stabilization of hyperexcitable neuronal membranes and inhibition of repetitive neuronal firing. Additionally, (R)-Lacosamide is also under clinical trials for the treatment of neuropathic pains.

Prior Art Methods of Synthesis of (R)-Lacosamide

Several methods and Schemes have been developed and disclosed in prior art, for the preparation of lacosamide. In general, most of the prior art approaches employed chiral pool approach and start from unnatural amino acid D-serine and its derivatives which are expensive. Methylation is carried out using Kuhn O-methylation which occurs in the presence of expensive catalyst viz. Ag₂O and is commercially not viable due to its high cost, non-regenerability of catalyst and longer reaction time (3-5 days) (Ref: Kuhn, R. Angew. Chem., Int. Ed. Engl. 1962, 1, 19).

Very recently, Sebastian Stecko reported the total synthesis of employed Lacosamidestereospecific allylcyanate-to-isocyanate rearrangement, which proceeds with chirality transfer starting from ethyl L-lactate (Ref: Stecko, S. J. Org. Chem. 2014, 79, 6342-6346.). However, this approach too suffers from the limitations of long reaction times and cumbersome steps. Limitations of the prior art have been overcome in the process of the present invention which discloses a new, general and highly efficient synthetic approach for the preparation of (R)-Lacosamide. The innovative process of the present invention for the synthesis of (R)-Lacosamide is described in description to the drawings and also illustrated in FIG. 1. The method is overall simple, flexible and highly efficient. The overall yield for (R)-Lacosamide by method of present invention as illustrated in FIG. 1, is about 52% in five steps (a, b, c, d and e). The method of present invention offers the following significant technical advantages:

-   -   i. high enantio-selectivity     -   ii. high yielding reaction steps     -   iii. protection free synthesis     -   iv. catalyst regenerability and     -   v. cost-effective strategy

A comparison of prior art methods with the method of the present invention is given in Table 1 below:

TABLE 1 Comparison of prior art methods with present invention Ser. patent Summary of Comparison with No. application No. prior art invention present invention 1. WO 2011130615 A3 This synthesis employs Method of present invention chiral pool approach and employs asymmetric synthesis. starting material is Starting material is cheap which is unnatural and expensive easily available racemic material- amino acid ‘D-serine’ butadiene monoepoxide. 2. WO 2011158194 This synthesis employs No enzymatic resolution needed or enzymatic resolution of involved. Direct synthesis of (R)- racemic Lacosamide, Lacosamide takes place. No loss which leads to 50% loss of of material occurs. material. 3. US 20130030216 Method uses D, L-Serine No diastereomeric resolution of and proceeds further with the mixture or precipitation of the diastereomeric resolution (R)-enantiomer involved at all. of the mixture to Overall synthesis occurs in just precipitate the (R)- five steps, starting from racemic enantiomer. Overall this butadiene monoepoxide. synthesis occurs in seven steps. 4. US 20130317109 Method employs No enzymatic acetylation and enzymatic acetylation and resolution involved. Direct resolution. enantioselective synthesis of (R)- Lacosamide takes place. 5. US 20130190533 Method involves use of No hydrogenation involved at all. catalytic asymmetric Process is thus safe and cost hydrogenation, which effective. requires hydrogen gas at high pressure and poses safety challenges besides increasing cost of process. 6. WO 2013072936 A2 Method involves Method does not involve protection and protection and deprotection of deprotection of amine amine moiety or any other moiety, which enhances group.Therefore, it is cost- cost of synthesis. effective. 7. WO 2011144983 Method employs N- Method does not employ any kind protected D-serine chiral of protection and deprotection of material and N- amine moiety or any other group. deprotection, which Instead it uses asymmetric enhances cost of synthesis. synthesis starting from the racemic material butadiene monoepoxide. 8. WO 2011095995 Method employs chiral Method does not use D-Serine as pool approach and starts the starting material and neither from D-serine followed by employs any kind of protection Boc protection of the and deprotection of amine moiety amine moiety and or any other functional group. deprotection of Boc group Rather method uses asymmetric using strong acid. synthesis, starting from racemic material butadiene monoepoxide. 9. WO 2011039781 A1 Method uses chiral pool Method neither use D-Serine as approach, starting from D- the starting material nor employs serine followed by any kind of protection and protection/deprotection of deprotection of amine moiety or the amine moiety and any other functional group. Rather hydroxyl group. method uses asymmetric synthesis, starting from racemic material butadiene monoepoxide. 10. WO 2013072936 A2 Method employs chiral Method neitheremploys chiral pool approach and Started pool approach nor does it use D- from D-serine followed by Serine as the starting material. Boc protection of the Further, it does not employ any amine moiety and kind of protection and deprotection of Boc group deprotection of amine moiety or using strong acid. any other functional group. Rather method uses asymmetric synthesis, starting from racemic material butadiene monoepoxide. 11 . WO 2013072330 A1 Method uses No protection/ de-protection of the protection/deprotection of amine moiety involved. No need the amine moiety followed for dia-stereoselective separation by dia-stereoselective for the pure (R)-Lacosamide. separation for the pure Overall synthesis occurs in just (R)-Lacosamide, in five steps with excellent yield. multisteps. 12. WO 2013030654 A1 Method uses chiral pool The present invention employs approach and starting from asymmetric synthesis. Further, it D-serine followed by does not employ any kind of benzyl protection of the protection and deprotection of amine moiety and amine moiety or any other hydrogenolysis of benzyl functional group. No group using hydrogen gas hydrogenation is involved at all. at high pressure. Process is thus safe and cost effective. 13. WO 2014155264 A1 This invention employs Chiral pool approach is not chiral pool approach and involved. Rather method uses uses D-serine as starting asymmetric synthesis, starting material followed by from racemic material butadiene protection/deprotection of monoepoxide. Method involves the amine moiety. The just 5 steps instead of seven. methodinvolves seven steps. 14. US 20140012044 A1 Method uses expensive Our invention does not employ and chiral starting material any kind of protection and benzyl glycidyl ether and deprotection of amine moiety or involves Boc protection/ any other group. Our invention deprotection of the amine also does not employ any moiety using strong acid. hydrogenation process at high Also, method involves pressure so is cost effective and hydrogenation which safe for the industrial production. requires hydrogen gas at high pressure.

DESCRIPTION OF THE DRAWINGS

FIG. 1 A: Structure of functionalized amino acids (FAAs) B : Structure of (R)-Lacosamide FIG. 2 Describes synthesis of (R)-Lacosamide from racemic butadiene monoepoxide by means of Dynamic Kinetic Asymmetric Transformation (DYKAT). The reagents used and the conditions of the reaction for each step, are as below: Step a: Phthalimide, Na₂CO₃, 1.2 mol % (R,R)-DACH, 0.4 mol % [η³-C₃H₅PdCl]₂, dry CH₂Cl₂, Room Temperature, 14 hours, about 98% yield Step b: MeI, NaH, DMF, 0° C. to Room Temperature, 3 hours, about 86% yield Step c: Two step reaction in one pot (i) OsO₄, NaIO₄, 2,6-lutidene, dioxane:water (3:1), 0° C. to Room Temperature, 3 hours. (ii) Oxone, DMF, Room Temperature, 3 hours. About 78% yield over two steps. Step d: C₆H₅CH₂NH₂, NMM, IBCF, THF, −78° C. to Room Temperature, 1 hour, about 88% yield Step e: Two step reaction in one pot (i) NH₂NH₂ • H₂O, isopropyl alcohol, 0° C. to Room Temperature, 2 hours. (ii) CH₃COCl, Na₂CO₃, dry toluene, 0 to 5° C., 1 hour. About 91% yield over two steps.

OBJECTS OF THE PRESENT INVENTION

The primary object of the present invention is to disclose a simple and cost-effective method for the asymmetric synthesis of the anti-epileptic drug (R)-lacosamide from racemic butadiene monoepoxide as starting material.

Another object of the present invention is to disclose a method for the asymmetric synthesis of antiepileptic drug (R)-lacosamide which has high enantio-selectivity with high yielding reaction steps and catalyst regenerability.

One more object of the invention is to disclose an improved method for the synthesis of Lacosamide which does not involve use of any protection/de-protection strategies, strong acids or hydrogenation, so is cost-effective.

Abbreviations

-   i. (R,R)-DACH: (1R,2R)-(+)-1,2-Diaminocyclohexane-N,N′-bis(2     dinaphthylphosphinobenzoyl) -   ii. DYKAT: Dynamic Kinetic Asymmetric Transformation -   iii. THF: Tetrahydrofuran -   iv. OsO₄: Osmium tetroxide -   v. DMF: Dimethylformamide -   vi. NaIO₄: Sodium periodate -   vii. NMM: N-Methylmorpholine -   viii. IBCF: Isobutylchloroformate -   ix. (R)—R isomer or R enantiomer -   x. (S)—S isomer or S enantiomer

SUMMARY OF THE PRESENT INVENTION

The present invention is directed towards an improved method for the preparation of the anti-epileptic drug lacosamide (FIG. 1B). The active form of the drug is (R)-enantiomer and the present method gives high yields of (R)-enantiomer of lacosamide. Unlike prior art methods, the improved method of present invention does not involve use of any protection/deprotection strategies, strong acids or hydrogenation. The synthetic route involved in the improved method is given in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The synthesis of (R)-Lacosamide is started from commercially available racemic butadiene monoepoxide and is described in FIG. 2, Steps a to e.

Butadiene monoepoxide can be easily synthesized using silver-catalyzed oxidation of 1,3-butadiene (Ref: Monnier, J. R.; Muehlbauer, P. J. Patent US 508096, Published 1992; Chem. Abstr. 1992, 116, 11439; U.S. Pat. No. 5,138,077, 1992)

The five steps, Step a to Step e, involved in the synthesis of (R)-Lacosamide by the process of the present invention, are as given below:

Step a: De-Racemisation

Deracemisation of butadiene monoepoxide (FIG. 2, No. 2) with palladium catalyzed Trost's Dynamic Kinetic Asymmetric Transformation (DYKAT) in the presence of chiral ligand 1.2 mol % (R,R)-DACH and 0.4 mol % [η³-C₃H₅PdCl]₂, phthalimide and base Na₂CO₃ afforded asymmetric allylic alkylation (AAA) product phthaloyl alcohol (FIG. 2, No. 3) as a single enantiomer with about 98% yield with ≧99% ee {[α]_(D) ²⁵ −72.2 (c 2.02, CH₂Cl₂) [Lit. −72.2 (c 2.02, CH₂Cl₂) (Ref: Trost, B. M.; Bunt, R. C.; Lemoine, R. C.; Calkins, T. L. J. Am. Chem. Soc. 2000, 122, 5968-5976).

Conditions and Yield:

Phthalimide, Na₂CO₃, 1.2 mol % (R,R)-DACH, 0.4 mol % [η³-C₃H₅PdCl]₂, dry CH₂Cl₂, Room Temperature, 14 hours, about 98% yield.

Step b: Methylation

The enantiomerically pure alcohol (FIG. 2, No. 3) was then subjected to O-methylation with MeI in presence of NaH which afforded methyl ether (FIG. 2, No. 4) in 86% yield.

Conditions and Yield:

MeI, NaH, DMF, 0° C. to Room Temperature, 3 hours, about 86% yield.

Step c: Oxidative Cleavage and Acid Formation:

The next step was to carry out amide formation at terminal double bond site. For this, the compound methyl ether (FIG. 2, No. 4) was subjected to oxidative cleavage in the presence of OsO₄ and sodium periodate followed by oxidation with oxone at room temperature, which led to formation of phthaloyl acid (FIG. 2, No. 5).

Conditions and Yield:

The reaction is carried out as two step reaction in one pot:

-   -   (i) OsO₄, NaIO₄, 2,6-lutidene, dioxane:water (3:1), 0° C. to         Room Temperature, 3 hours     -   (ii) Oxone, DMF, Room Temperature, 3 hours (about 78% yield over         two steps).         Step d: Amide Formation:

The treatment of phthaloyl acid (FIG. 2, No. 5) with benzyl amine in presence of isobutylchloroformate and N-methyl morpholine in THF at −78° C. afforded the phthaloyl amide (FIG. 2, No. 6) in about 88% yield.

Conditions and Yield:

C₆H₅CH₂NH₂, NMM, IBCF, THF, −78° C. to Room Temperature, 1 hour, about 88% yield.

Step e: Phthalimide Cleavage and Acetylation:

Finally, the cleavage of phthalimide group of phthaloyl amide (R) (FIG. 2, No. 6) with hydrazine hydrate in presence of isopropyl alcohol followed by N-acetylation using acetyl chloride under basic conditions furnished the target compound(R)-Lacosamide (FIG. 1B) in about 91% yield {[α]_(D) ²⁵ +16.1 (c 1, MeOH) [Lit. +16.2 (c 1, MeOH),^(5g) +16.1 (c 1.2, MeOH)^(5h)]}. The spectroscopic and physical data of (R)-Lacosamide (FIG. 1B) were in full agreement with literature data.

Conditions and Yield:

The reaction is carried out as a two step reaction in one pot:

(i) NH₂NH₂.H₂O, isopropyl alcohol, 0° C. to Room Temperature, 2 hours

(ii) CH₃COCl, Na₂CO₃, dry toluene, 0 to 5° C., 1 hour. About 91% yield over two steps

Illustration of Technical Aspects

In the following section, technical aspects are described by way of examples to illustrate the process of the invention. However, these do not limit the scope of the present invention. Several variants of these examples would be evident to persons ordinarily skilled in the art.

All reactions were carried out under argon or nitrogen in oven-dried glassware using standard glass syringes, cannulas and septa. The solvents were dried and purified by standard procedures prior to use. Infrared spectra (IR) were run on Agilent resolution Pro 600. Optical rotations were obtained on automatic polarimeter AA-65. ¹H NMR and ¹³C NMR spectra were obtained at 400 MHz and 100 MHz respectively on JEOL ECS-400 MHz spectrometer using TMS as internal reference standard. The reactions were monitored on TLC plates Merck silica gel 60 F254 and visualization with UV light, ninhydrin, anisaldehyde in ethanol and 2,4-Dinitrophenylhydrazine as developing reagents. High resolution Mass spectra (HRMS) were recorded on Agilent 6530 accurate-mass Q-TOF using electron spray ionization.

EXAMPLE 1 Preparation of (S)-2-(Isoindolin-2-yl)but-3-en-1-ol (FIG. 2, No. 3)

A mixture of π-allylpalladium chloride dimer 0.4 mol % (20 mg, 53 μmol), 1.2 mol % (R,R)-DACH ligand (125 mg, 158 μmol, Na₂CO₃ (70 mg, 0.66 mmol) and phthalimide (1.94 g, 13.2 mmol) in 100 mL of dry CH₂Cl₂ was purged with nitrogen for 1 h. The resulting mixture was stirred for 10 min at room temperature to which butadiene monoepoxide (FIG. 2, No. 2) (920 mg, 13.2 mmol) was added. The resulting mixture was stirred at room temperature under nitrogen for 14 h, concentrated in vacuo and purified by silica gel column chromatography using ethyl acetate/hexane (3:7) as eluent furnished 2.8 g (98%) yield of (S)-2-(Isoindolin-2-yl) but-3-en-1-ol (FIG. 2, No. 3) as a crystalline white solid. [R_(f)=0.21, ethyl acetate/hexane (3:7)]; [α]_(D) ²⁵ −72.2 (c 2.02, CH₂Cl₂) [Lit. −72.2 (c 2.02, CH₂Cl₂)]; IR (CH₂Cl₂) v: 3467, 1773, 1698, 1467, 1383 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 2.7 (bs, 1H), 3.9 (m, 1H), 4.1 (m, 1H), 4.9 (m, 1H), 5.2 (m, 2H), 6.1 (ddd, J=6.4 Hz, 1H), 7.7 (dd, J=3.2, 5.48 Hz, 2H), 7.8 (dd, J=2.76, 5.48 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 55.9, 62.8, 118.8, 123.4, 131.7, 131.9, 134.2, 168.5. (Ref: Trost, B. M et al. J. Am. Chem. Soc. 2000, 122, 5968-597).

EXAMPLE 2 Preparation of (S)-2-(1-Methoxybut-3-en-2-yl)isoindoline (FIG. 2, No. 4)

To a solution of (S)-2-(Isoindolin-2-yl) but-3-en-1-ol (FIG. 2, No. 3)(2.0 g, 9.2 mmol) in 40 mL DMF was successively added NaH (442 mg, 18.4 mmol) at 0° C., stirred for ten minutes followed by addition of MeI (1.73 mL, 27.6 mmol) and then the reaction mixture was stirred at room temperature for 3 h. The reaction mixture was quenched by addition of ice cold water, extracted with diethyl ether, washed with brine and dried over anhydrous MgSO₄. The organic layer was then concentrated in vacuo and purified by silica gel column chromatography using ethyl acetate/hexane (1:9) as eluent to furnish 1.83 g (86%) yield of (S)-2-(1-Methoxybut-3-en-2-yl) isoindoline (FIG. 2, No. 4) as a crystalline white solid. [R_(f)=0.56, ethyl acetate/hexane (3:7)]; [α]_(D) ²⁵ −75.1 (c 1.0, CH₂Cl₂); IR (CH₂Cl₂) v: 1773, 1708, 1468, and 1384 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ:3.3 (s, 3H), 3.6 (m, 1H), 4.06 (m, 1H), 5.0 (m, 1H), 5.22-5.31 (m, 2H), 6.12 (ddd, J=7.32 Hz, 1H), 7.68 (dd, J=2.76, 5.48 Hz, 2H), 7.80 (dd, J=2.76, 5.48 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 52.8, 58.7, 71.3, 119.0, 123.2, 131.9, 132.1, 133.9, 168.0; HRMS (ESI-TOF) m/z calcd for C₁₃H₁₃NO₃Na [M+Na⁺] 254.080. Found 254.079.

EXAMPLE 3 Preparation of (R)-2-(Isoindolin-2-yl)-3-methoxypropanoic acid (FIG. 2, No. 5)

To a solution of compound (S)-2-(1-Methoxybut-3-en-2-yl) isoindoline (FIG. 2, No. 4)(1.5 g, 6.5 mmol) indioxane-water (3:1, 40 mL) was added 2,6-lutidine (1.5 mL, 13 mmol), OsO₄ (0.1 M solution in toluene, 1.3 mL, 0.13 mmol) and NaIO₄ (2.78 g, 13 mmol). The reaction was stirred at 25° C. for 3 h. After completion of reaction, water (10 mL) and CH₂Cl₂ (30 mL) were added. The organic layer was separated, and the water layer extracted with CH₂Cl₂ (3×10 mL). The combined organic layer was washed with brine and dried over anhydrous MgSO₄, concentrated in vacuo to give crude aldehyde which was used as such for the next step without further purification.

The above aldehyde was dissolved in DMF and oxone (2 g, 6.5 mmol) was added in one portion and stirred at room temperature for 3 h. The resulting solution was diluted with water, filtered through a celite pad, washed and extracted with diethyl ether (3×20 mL). The organic extract was washed with brine, dried over anhydrous MgSO₄, and the solvent was removed in vacuo to obtain the crude product (R)-2-(Isoindolin-2-yl)-3-methoxypropanoic acid (FIG. 2, No. 5) (1.26 g, 78%, yield determined by ¹H NMR) which was used as such for the next step without further purification due to its more polar nature. The analytical sample was obtained by preparative chromatography on silica gel (CH₂Cl₂:MeOH, 9:1) as yellow oil. [α]_(D) ²⁵ +66.5 (c 0.1, MeOH); IR (CH₂Cl₂) v: 2896, 1775, 1699, 1604 and 1392 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ:3.36 (s, 3H), 4.0 (m, 1H), 4.17 (t, J=10.08 Hz, 1H), 5.17-5.20 (m, 1H), 7.74 (dd, J=3.2, 5.52 Hz, 2H), 7.86 (dd, J=3.2, 5.52 Hz, 2H), 8.05 (bs, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 56.0, 62.9, 118.9, 123.5, 131.8, 132.0, 134.2, 168.6; HRMS (ESI-TOF) m/z calcd for C₁₂H₁₁NO₅Na [M+Na⁺] 272.050. Found 272.05.

EXAMPLE 4 Preparation of (R)—N—Benzyl-2-(isoindolin-2-yl)-3-methoxypropanamide (FIG. 2, No. 6)

To the crude acid (R)-2-(Isoindolin-2-yl)-3-methoxypropanoic acid (FIG. 2, No. 5)(1.25 g, 5 mmol) in dry THF was added N-methylmorpholine (0.66 mL, 6.0 mmol) at −78° C. under an argon atmosphere. After 5 min, isobutyl chloroformate (0.78 mL, 6.0 mmol) was added and stirred for another 5 min. To this reaction mixture benzylamine (0.65 mL, 6.0 mmol) was added at −78° C. after which the reaction mixture was stirred at room temperature for 1 h. After completion of the reaction, the reaction mixture was filtered through a celite pad, washed with ethyl acetate, and dried over anhydrous MgSO₄. The solvent was removed in vacuo and the crude product was subjected to silica gel column chromatography (Ethyl acetate/Hexane, 4:6) to yield 1.48 g, 88% yield of (R)—N—Benzyl-2-(isoindolin-2-yl)-3-methoxypropanamide (FIG. 2, No. 6) as a crystalline solid. [R_(f)=0.26, ethyl acetate/hexane (4:6)]; [α]_(D) ²⁵ +81.8 (c 1, CH₂Cl₂); IR (CH₂Cl₂) v: 1718, 1685, 1535, and 1387 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ:3.38 (bs, 3H), 3.75 (m, 1H), 4.34 (t, J=9.64 Hz, 1H), 4.46 (m, 2H), 5.0 (m, 1H), 7.1 (bs, 1H), 7.21-7.33 (m, 5H), 7.7 (dd, J=3.2, 5.48 Hz, 2H), 7.83 (dd, J=3.2, 5.48 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 29.6, 43.5, 51.7, 58.9, 70.1, 123.5, 127.1, 127.3, 127.4, 128.4, 128.6, 131.8, 134.1, 137.9, 167.3, 167.9; HRMS (ESI-TOF) m/z calcd for C₁₉H₁₈N₂O₄Na [M+Na⁺] 361.120. Found 361.116.

EXAMPLE 5 Preparation of the Final Product (R)-Lacosamide (FIG. 1, No. B)

To a solution of the compound (R)—N—Benzyl-2-(isoindolin-2-yl)-3-methoxypropanamide (FIG. 2, No. 6) (1.4 g, 4.1 mmol) in 20 mL of isopropyl alcohol was added hydrazine monohydrate (0.22 mL, 4.5 mmol) at 0° C. under nitrogen atmosphere. The reaction was stirred at 25° C. for 2 h. The resulting solution was filtered, washed with diethyl ether, brine, dried over magnesium sulphate and concentrated in vacuo to furnish the crude compound [R_(f)=0.36, CH₂Cl₂:MeOH (9:1)]. The resulting crude was used as such for the next step without further purification.

The residue was then dissolved in dry toluene followed by addition of Na₂CO₃ (1.3 g, 12.3 mmol) was added. The reaction mixture was cooled to 0° C. after which acetyl chloride (0.33 mL, 4.5 mmol) was slowly added and the solution stirred at 5° C. for 1 h. After completion of the reaction, the solid was filtered through a celite pad and the solvent was evaporated in vacuo. The crude product was purified by silica gel column chromatography (CH₂Cl₂:MeOH, 95:05) to afford 935 mg (91%) yield of (R)-Lacosamide (FIG. 1, No. B) as white solid [R_(f)=0.47, CH₂Cl₂:MeOH (9:1)]; mp 143-144° C. [Lit. 140-141° C., 142-143° C.]; [α]_(D) ²⁵ +16.1 (c 1, MeOH) [Lit. +16.2 (c 1, MeOH), +16.1 (c 1.2, MeOH),]; IR (CH₂Cl₂) v: 3054, 2928, 1650, 1529, 1372, 1264, and 1118 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ:2.02 (s, 3H), 3.37 (s, 3H), 3.44 (m, 1H), 3.80 (m, 1H), 4.47 (m, 2H), 4.57 (m, 1H), 6.54 (s, 1H), 6.86 (s, 1H), 7.24-7.68 (m, 5H); ¹³C NMR (100 MHz, CDCl₃) δ: 23.2, 43.5, 52.3, 59.0, 71.6, 127.4, 127.4, 128.7, 137.8, 169.9, 170.3.

(Ref: Wadavrao et al. Synthesis 2013, 45, 3383-3386; Stecko, S. J. Org. Chem. 2014, 79, 6342-6346).

Novelty

The present invention discloses a new process for the synthesis of the anti-epileptic drug (R)-Lacosamide, which has not been disclosed in the prior art. Unlike prior art methods which use expensive starting materials e.g. unnatural amino acids such as D-Serine, the present method (FIG. 2, Steps a to e) uses a cheap and easily available racemic material-butadiene monoepoxide. Also, the process is completed in just five steps, does not involve use of any protection/deprotection strategies or use of any strong acid or hydrogenation.

Inventive Step

The technical advancement of knowledge, as disclosed in the present invention lies in synthesizing the active form of the anti-epileptic drug, (R)-Lacosamide using a new route as disclosed in FIG. 1, Steps a to e.

INDUSTRIAL APPLICATION

The drug (R)-Lacosamide is widely used for the treatment of epilepsy. The present invention discloses an easy, cost-effective process for its synthesis and thus has widespread industrial application. 

We claim:
 1. A process for synthesis of >99% ee enantio-pure (R)-Lacosamide from racemic butadiene monoepoxide and the said process comprising the five steps of:

i. Deracemisation: of butadiene monoepoxide 2 with palladium catalyzed Trost's DYKAT in the presence of chiral ligand 1.2 mol % (R,R)-DACH and 0.4 mol % [η³-C₃H₅PdCl]₂, phthalimide and base Na₂CO₃ at room temperature for period of 14 hours afforded asymmetric allylic alkylation (AAA) product phthaloyl alcohol

ii. Methylation: preparing methyl ether (S)-4 from phthaloyl alcohol (S)-3 with methyl iodide under basic conditions in solvent at temperature in the range of 0° C. to Room Temperature for period of 3 hours;

iii. Oxidative cleavage and acid formation: oxidative cleavage of the terminal double bond of (S)-4 in the presence of osmium tetraoxide and sodium periodate in solvent at temperature in the range of 0° C. to Room Temperature for period of 3 hours followed by oxidation with oxone in solvent at room temperature for period of 3 hours afforded (R)-5;

iv. Amide formation: amide formation of (R)-5 as obtained in step (iii) with benzyl amine in the presence of isobutylchloroformate and N-methylmorpholine in solvent at temperature in the range of −78° C. to Room Temperature for period of 1 hour 1 hour afforded (R)-6;

v. Phthalimide cleavage and acetylation: cleavage of phthalimide group of amide (R)-6 as obtained in step (iv) with hydrazine hydrate in solvent followed by N-acetylation in the presence of acetylating agent, base and solvent to obtain (R)-Lacosamide 1B.
 2. The process as claimed in step (i) of claim 1, wherein the base is a metal carbonate.
 3. The process as claimed in step (i) of claim 1, wherein the epoxide is allylic epoxide.
 4. The process as claimed in step (ii) of claim 1, wherein the base is selected from the group consisting of metal hydrides, hydroxides, oxides, carbonates, bicarbonates, alkoxides or primary and secondary amine, ammonia, alkyl amine.
 5. The process as claimed in step (iii) of claim 1, wherein the oxidizing agent is selected from the group consisting of PDC, PCC, KMnO₄, Oxone, NaClO₂.
 6. The process as claimed in step (iv) of claim 1, wherein temperature is in the range of −100° C. to Room Temperature.
 7. The process as claimed in step (v) of claim 1, wherein the acetylating agent used is selected from the group consisting of acetic anhydride, acetyl chloride, acetic acid and mixture thereof or suitable derivatives thereof.
 8. The process as claimed in step (v) of claim 1, wherein the base is selected from the group consisting of carbonates, bicarbonates, hydroxides, hydrides, alkoxides, aryl amines, aliphatic amines, heterocyclic compound and like thereof.
 9. The process as claimed in step (v) of claim 1, wherein solvent used is selected from group consisting of esters, ketones, aliphatic or aromatic hydrocarbons, acids, nitriles, water, aldehydes, alcohols, halides, non-polar solvents and mixture thereof.
 10. The process as claimed in claim 9, wherein solvent used is selected from the group consisting of toluene, methanol, ethanol, acetonitrile, THF, acetone, petroleum ether, n-hexane, isopropanol, acetic acid, ethyl acetate, dichloromethane, water, water miscible solvents or mixtures thereof.
 11. The process as claimed in claim 1, wherein yield of the said (R)-Lacosamide is 52%.
 12. The process as claimed in step (i) of claim 1, wherein the base is selected from the group consisting of Na₂CO₃, K₂CO₃.
 13. The process as claimed in step (i) of claim 1, wherein the epoxide is butadiene monoepoxide.
 14. The process as claimed in step (ii) of claim 1, wherein the base is a metal hydride.
 15. The process as claimed in step (iii) of claim 1, wherein the oxidizing agent is oxone.
 16. The process as claimed in step (iv) of claim 1, wherein temperature is in the range of −78° C. to Room Temperature. 